In the wet chemical metallization of polymer coatings such as in the manufacture of printed circuit boards, polymer swell and etch steps are often used to enhance metal-to-polymer adhesion. A commonly used etchant for this purpose is a mixture of chromic acid and sulfuric acid which, operating at elevated temperatures, readily attacks the previously swelled polymeric materials and generates a microroughened surface for rooting the metal to be deposited thereon.
In the past, the swell-etch procedure was used on bare polymer surfaces, i.e., surfaces which had no exposed copper or other conductor. More recently, with the advent of multilayer printed circuit boards, the need to etch the polymer surface in an environment which includes exposed metal conductors has arisen. This is due to processing sequences which include the application of patterned polymer dielectric films, such as photodefinable polymers, over pre-deposited conductors followed by a metallization step over the polymer film. When employing standard aqueous chromic acid-sulfuric acid etch solutions, we unexpectedly discovered that a deeply etched trench formed in the exposed conductor at the boundary with the dielectric film. These trenches, if sufficiently enlarged, could break through the metal film, causing openings in the circuit or, if not that deep, could increase the resistance of the conductor or otherwise adversely affect the physical properties of the conductor. It therefore became necessary to modify or change the etch solution to attain an etchant for the polymer which, during the etching operation, would either not etch the metal conductor or etch it relatively slowly without significant trenching.